Chafetz Chaim, Part Two, The Prohibition Against Rechilut, Principle 3
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וְאִם כַּוָּנָתוֹ בְּסַפְּרוֹ אֶת הַדָּבָר לַחֲבֵרוֹ, כְּדֵי שֶׁיּוֹכִיחַ אֶת רְאוּבֵן עַל שֶׁדִּבֵּר לָשׁוֹן הָרָע עַל שִׁמְעוֹן, עַיֵּן לְעֵיל בְּחֵלֶק א' בִּכְלָל י' סָעִיף ו'.
Chofetz Chaim, Silverstein translation
And if it were his intent in relating this to his friend to have him rebuke Reuven for speaking lashon hara against Shimon, see above, Part One, Principle X, section 6.